H 1 H 2 Primary Secondary X 1 X 2 Time Primary voltage Secondary voltage Time Voltage primary to secondary Volts Volts 10 Distribution Transformer Handbook
Additive Polarity In the additive transformer shown above, the windings are in opposite directions. The upper graph shows the primary voltage and secondary voltage, with both measurements taken left-to-right. The voltages are 180º out-of-phase. The lower graph shows the voltage difference between the two graphs. The voltage between them is more than the primary voltage, as indicated by the shaded areas. The waveforms add. The transformer has additive polarity. The secondary bushings are numbered right-to-left. Single-phase transformers below 200 kVA with primary voltage below 8600 volts, usually have additive polarity. All other single-phase transformers usually have subtractive polarity. H 1 H 2 Primary Secondary X 2 X 1 Time Time Primary voltage Secondary voltage Voltage primary to secondary Volts Volts Transformer Concepts 11
12 Distribution Transformer Handbook These rules apply to all transformers, regardless of polarity: • H 1 is the left primary bushing. • What goes into H 1 goes out of X 1 – the voltage at the X 1 bushing is in-phase with the voltage at the H 1 bushing. POLARITY TEST Polarity is listed on the transformer nameplate. If in doubt, this test will determine the polarity of a transformer: 1. Connect two adjacent terminals of the high and low voltage windings. 2. Apply a moderate (120 volts) voltage across the high voltage terminals. Do not apply the 120 volts to the secondary terminals. This will induce a lethal voltage across the primary terminals. 3. Measure the voltage across the other high and low voltage terminals. 4. The polarity is additive if the measured voltage is higher than the applied voltage. The polarity is subtractive if the measured voltage is lower than the applied voltage. Jumper = AC voltage source = Voltmeter M M
Transformer Concepts 13 WYE, DELTA CONFIGURATIONS Three transformer windings can be configured as a delta or a wye, to deliver three-phase services. Delta One end of a coil is plus and the other end is minus. To make a delta connection, connect unlike markings of each coil together. Line-to-line voltage is the same as the voltage across a transformer winding. Line current is 1.73 times the current in a transformer winding. Delta configurations: Delta Center-grounded delta Corner-grounded delta Open delta V L-L V L-L V L-L + + + – – –
14 Distribution Transformer Handbook Wye One end of a coil is plus and the other end is minus. To make a wye connection, connect like markings of each coil together. The remaining terminals are the output terminals. Line-to-line voltage is 1.73 times line-to-neutral voltage (the voltage across a transformer winding). Line current is the same as the current in a transformer winding.
You've reached the end of your free preview.
Want to read all 86 pages?
- Fall '19
- Transformer Concepts